This invention relates to fuel cells and, in particular, to an electrolyte matrix for use in molten carbonate fuel cells.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode and a cathode separated by an electrolyte, which serves to conduct electrically charged ions. In order to produce a useful power level, a number of individual fuel cells are stacked in series with an electrically conductive separator plate between each cell.
Molten carbonate fuel cells (“MCFCs”) operate by passing a reactant fuel gas through the anode, while oxidizing gas is passed through the cathode. The anode and the cathode of MCFCs are isolated from one another by a porous electrolyte matrix which is saturated with carbonate electrolyte. Molten carbonate fuel cell performance and operating life are dependent in part on the characteristics of the anode and the cathode employed in the fuel cell. For example, fuel cell anodes must have a sufficient capacity for electrolyte retention, high mechanical strength and durability.
Most commonly used fuel cell anodes comprise a nickel (Ni) based alloy, such as Ni—Cr or Ni—Al. Aluminum and chromium are commonly used to improve the mechanical strength and durability of the anode so as to reduce creeping of the anode electrode and to improve its wettability. For example, U.S. Pat. No. 4,714,586 discloses preparation of Ni—Cr anodes for use in MCFCs by uniformly mixing Ni and Cr powders, sintering the Ni and Cr mixture at 1000-1100° C. to produce a porous Ni—Cr plaque and exposing the plaque to an oxidizing environment at elevated temperatures to sufficiently oxidize the chromium in the alloy. Although the resulting Ni—Cr alloy anodes exhibit high creep resistance, these anodes nevertheless exhibit reduced performance due to development of an excessive number of sub-micron pores in the anode during the first 200 hours of operation. The presence of sub-micron pores in the anode enhances electrolyte retention within the anode. However, development of an excessive number of sub-micron pores can result in electrolyte flooding due to excessive storage of electrolyte in the pores which can cause an increased gas diffusion resistance. Moreover, testing of sintered Ni—Cr anodes after about 4000 hours of operation in MCFCs showed that these electrodes can lose their strength and become soft.
In contrast, anode electrodes formed from Ni—Al alloys have been shown to exhibit improved wettability and high creep resistance. Also, anodes formed from Ni—Al alloys do not develop sub-micron pores during operation in MCFCs, and therefore have a significantly lower electrolyte storage capacity than the Ni—Cr anodes. Insufficient electrolyte retention in the anode can increase interface resistance between the anode and the electrolyte matrix and can negatively affect the performance of the fuel cell, which, in turn, can have a negative effect on the operating life of the MCFC.
U.S. Pat. No. 5,415,833 discloses a further anode formed from a Ni—AlCr alloy to realize a reduced amount of creep. In the method of this patent several heat treatments at elevated temperatures, e.g. 800° C., and special atmospheres were used to fabricate the electrode. The patent also mentions the use of about 6% combined Al and Cr in the Ni—AlCr anode, with the ratio of Al to Cr being 4 to 1.
Accordingly, an anode having an optimized pore structure to enhance electrolyte retention in the pores of the anode without affecting gas diffusion into the pores is desired. In addition, a method for manufacturing such anodes that is simple and does not require multiple steps to be performed at increased temperatures is also desired, so as to reduce manufacturing costs and to avoid brittleness of the anode resulting from the use of high temperatures.
It is therefore an object of the present invention to provide an anode with an improved pore structure which allows for sufficient electrolyte retention without causing electrolyte flooding and reducing gas diffusion into the pores of the anode.
It is a further object of the present invention to provide a method of preparing an anode with improved pore structure which does not require heating or sintering of the anode at high temperatures.
It is also an object of the present invention to provide a method of preparing an anode with improved pore structure which is simple to perform and is cost effective.